Submission for mod-protocols-tcp-ip


Marvin Solomon (uwvax!crys.wisc.edu!solomon@seismo.CSS.GOV)
26 Mar 87 17:19:24 GMT


Path: crystal!solomon
From: solomon@crys.WISC.EDU (Marvin Solomon)
Newsgroups: mod.protocols.tcp-ip
Subject: Re: Station wagon full of bits
Summary: There's more to data communication than bits/second.
Message-ID: <291@crys.WISC.EDU>
Date: 26 Mar 87 17:19:24 GMT
References: <VAX-MM(195)+TOPSLIB(124).25-Mar-87.20:55:50.VAX.DARPA.MIL>
Organization: U of Wisconsin CS Dept
Lines: 31

Vint Cerf has pointed out one pitfall in trying to use a single number
to compare very different technologies: One must consider not only
bandwidth, but also latency. But all the discussion on this topic so
far has failed to note that the information-carrying capacity of a
station wagon full of tapes (or a knapsack full of CS's, or whatever) and a
56Kbps leased line have different dimensions! Vint's "Johnny Appleseed"
has units capacity*velocity = bits*length/time, whereas a communications line
is measured in bits/time. Thus, to compare the two, one has to mention the
distance. The problem we are considering was posed by Jon Bentley in his
delightful Programming Pearls column, "The Back of the Envelope" in the March
1984 issue of "Communications of the ACM":

        At what distances can a courier on a bicycle with a reel of magnetic tape
        be a more rapid carrier of information than a 56-kilobaud [sic] telephone
        line? Than a 1200-baud line? [op cit, p. 182]

The answers (based on considerably more realistic estimates about magnetic
tape than have been bandied about in this forum, by the way) are,
respectively, 20 miles and the distance the cyclist can travel in a week.
The point is that a communication line can beat ANY bulk transfer over
sufficiently long distances. Of course, we can effectively cut off discussion
at (say) the diameter of the earth.

To summarize: The advantages of networking are both low latency and
distance-independence.

--
	Marvin Solomon
	Computer Sciences Department
	University of Wisconsin, Madison WI
	solomon@gjetost.wisc.edu or seismo!uwvax!solomon



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